1. Field of the Invention
The present invention relates to a liquid crystal display device and a substrate, and more particularly to a liquid crystal display device which can make gradational display.
2. Description of the Related Art
Liquid crystal displays are flat and low power consumers and are extensively used for notebook computers. One of remarkable features of the liquid crystal display device is its particularly low power consumption as compared with other display devices such as CRTs and plasma displays, and its future application to portable information-processing equipment is expected.
Such portable equipment is desired to have a display which consumes low power of 500 mW or below, and preferably several mW. In response to such a demand, a reflective type liquid crystal display device, which is a simple matrix type using a TN (twisted nematic) liquid crystal, does not need a back light and has only a low power consumption, has been used conventionally. But, this display has a disadvantage that since the TN type needs a polarizing plate, its reflectance is about 30% and dark. Besides, the simple matrix type has a disadvantage that when the number of pixels is increased, contrast is lowered, and display image quality is deteriorated. Therefore, using a PCGH (phase change guest host type) mode liquid crystal not requiring to use a polarizing plate and driving by active matrices, it has been attempted to produce a display provided with a high reflectance and a high contrast.
FIG. 1 shows one example of the circuit structure of a pixel in a conventional liquid crystal display device. This circuit structure of the pixel shown in FIG. 1 is the same as a conventional transmission active matrix liquid crystal display device. When a thin film transistor 1301 is turned on by a scan signal applied to a gate line 1304, a display signal voltage applied to a data line 1305 is applied to a liquid crystal layer 1303. And, an electrical charge is applied to a storage capacitance 1302 by a storage capacitance line (Cs line) 1307. As it is known well, it is generally necessary to apply an alternating voltage to the liquid crystal layer 1303, and the display signal voltage, which becomes positive or negative with a voltage of an opposed electrode 1306 formed on an opposite substrate at the center, is applied to the signal line 1305 to drive the pixel.
Such a liquid crystal display device needs the application of an alternating voltage to the liquid crystal layer even when the display does not change at all.
Therefore, a pixel potential is renewed whenever selected by frame cycling. Power consumption P when an alternating voltage is applied to the capacitance is expressed as follows: EQU p=f.times.V.sup.2 .times.C
where, f denotes a frequency, V a voltage, and C capacitance. Therefore, the higher the frequency, the higher the voltage or the higher the capacitance, the higher the power consumption becomes.
For the AC drive of the liquid crystal display device, the drive frequency of each pixel is a frame frequency, the drive frequency of a data line is the product of a frame frequency and the number of scan lines, the drive frequency of a data line driver IC is the product of the number of all pixels on the screen and a frame frequency, and, if a division drive is performed, it is a value obtained by further dividing by a division number. For example, in a diagonally 10.4-inch color VGA (640.times.RGB.times.480 pixels) liquid crystal display device, a data line IC has power consumption of about 1 W. Therefore, an A4-size 150 dpi-equivalent high definition liquid crystal display device has the number of pixels of about 1600.times.1200 6.25 times greater than VGA, resulting in a disadvantage that power consumption is so high as 2 to 3 W or more. When a high power-consuming liquid crystal display device is used for any portable information-processing equipment, there are disadvantages that a battery exhausts soon and the usable duration is shortened.
On the other hand, it is known that power consumption can be reduced by using a bistable ferroelectric liquid crystal (SSFLC). Since the ferroelectric liquid crystal has a memory, the voltage supply can be stopped if the screen is not changed. But, the bistable ferroelectric liquid crystal has a disadvantages in that its orientation is disturbed by an impact, resulting in a screen failure. Therefore, it cannot be used as a portable display device. Besides, the liquid crystal having memory often has its contrast and reflectance limited, and its display has a problem in quality. For example, it concerns a display mode, which needs a polarizing plate, of the SSFLC, and the screen is dark with the reflectance of about 30%. Besides, since what is displayed by the SSFLC is basically limited to a binary display because it is bistable, its presentation capacity (namely, a volume of information) is lowered to great extent as compared-with the display mode which can make gradation display. This constitutes a very serious problem in making a color display. When spatial light modulation is made by a dither method or the like to make the gradation display, effective resolution is degraded. Besides, when temporal modulation is made by frame rate control or the like, a flicker takes place, thus the SSFLC cannot be applied to animations.
On the other hands, conventional liquid crystal display devices generally obtain halftone display by changing the transmittancy of liquid crystal with a voltage applied to the liquid crystal. As compared with other methods, the liquid crystal display device has a relatively simple structure and drive method, and can make full color display by applying a voltage in an analog fashion.
Halftone or gray scale display by controlling the voltage applied to the liquid crystal can be applied to, for example, a TN liquid crystal which has a moderate threshold property. But, it cannot be applied to a liquid crystal which has a sharp threshold property and a good binary displaying property, such as a ferroelectric liquid crystal and an antiferroelectric liquid crystal.
In response to the demands for a liquid crystal display device having a wide angle of visibility and a quick response in these years, a ferroelectric liquid crystal and an antiferroelectric liquid crystal which are good in binary displaying property are attracting attention.
Since the ferroelectric liquid crystal has a sharp threshold property, it is suitable for a binary display but not suitable for a halftone display. Therefore, various studies have been made to provide a halftone display.
Some of such studies are based on the limit of a spatial resolving power of human eyes, including a dither method by which a single pixel is spatially divided into a plurality of pixels and a binary display is made by the respective divided pixels to make a gradation display, and a field thinning method by which a temporal ratio between a light state and a dark state is controlled to make a gradation display.
And, an attempt has been made to make a gradation display with a voltage applied by varying the optical response of a liquid crystal material itself to make a threshold value smooth.
However, the application of such existing techniques to a binary liquid crystal display device having an excellent displaying performance is not effective to achieve a gradation display at a practical level.
When the liquid crystal material itself is modified to make a threshold property moderate, there is a disadvantage for a ferroelectric liquid crystal, for example, that good properties such as a high-speed response and a memory are deteriorated. And, there is another disadvantage that the stability of a liquid crystal material is deteriorated.
And, when the halftone display is made according to a drive method such as a field thinning method or the like, a volume of data to be sent to each frame becomes enormous with the increase of the number of gradations, and the treatment is limited. Besides, there is another disadvantage of causing a flicker.
Since the dither method divides a single pixel into a plurality of very small pixels, it has disadvantages that the production of a liquid crystal display device is subdivided, resulting in lowering a yield and making the production process complicated. Besides, since it is necessary to drive the respective very small pixels, the drive system is also complicated heavily.
Furthermore, the number of gradations which can be displayed depends on the divided number of a single pixel, so that additional division is required to increase the number of gradations, and an analog full-color display is essentially impossible.
As described above, the displays for a personal computer and portable information-processing equipment show mostly still images, and an alternating voltage is supplied to a data line even when the screen is not changed, resulting in waste of power.
In view of the circumstances, the present invention aims to remedy the above-described disadvantages and to provide a liquid crystal display device which does not consume much power.
And, the invention aims to provide a liquid crystal display device which enables gradation display and provides high resolution, high image quality display.
The invention has been completed to remedy the above-described disadvantages. Specifically, the invention aims to provide a liquid crystal display device which can make a gradation display without being limited by the threshold property of a liquid crystal material. And, the invention aims to provide a liquid crystal display device which can make a gradation display without deteriorating the remarkable properties such as a high-speed response and a memory of a liquid crystal having a sharp threshold property.
In addition, the invention aims to provide a liquid crystal display device which can make a gradation display without subdividing a pixel electrode.